The state of the art in head-up display (HUD) systems for aircraft cockpits is illustrated by such patents as U.S. Pat. No. 3,885,095 issued to G. Wolfson et al on May 20, 1975, U.S. Pat. No. 3,915,548 issued to E. W. Opittek et al on Oct. 28, l975, and U.S. Pat. No. 3,940,204 issued to R. J. Withrington of Feb. 24, 1976.
In recent years there has been a significant advance in the state of the art of components and techniques applicable to the pilot's display unit of a head-up display system. These advances include new equivalent source means for generating images through the use of liquid crystal or scanned laser techniques as well as improvements in the traditional cathode ray tube (CRT). Also, new techniques for designing and building the collimating optical system have been devised. The advent of diffraction optics technology has opened the way for more efficient wider field of view displays.
The major use of a HUD in a high performance aircraft is to improve the pilot's capability for effective target acquisition and weapon delivery by providing him with a display of information essential for the performance of these tasks while allowing him to fly in a head-up mode, that is, seeing out of the cockpit. In order to be effective, the requirements established for any HUD system must consider the type and placement of information to be displayed as well as physical considerations such as field of view, viewability and installation. A larger field of view will significantly improve a HUD's utility for both target acquisition and weapon delivery. For target acquistion, the desirable maximum azimuth field of view is about 30 degrees while for weapon delivery an improved elevation field of view is desirable. Reasonable total elevation fields of view are 20 to 25 degrees. There may, however, remain physical restrictions due to cockpit arrangment which limit the available field of view.
It is an object of the present invention to provide an optical system capable of providing at least these desirable fields of view.
Improving the instantaneous field of view over its present narrow limits (the traditional weak point of existing HUD designs) also provides the ability to declutter the display--an important aspect in any improved HUD.
Examination of possible approaches to the design of advanced HUDs has pointed to HUDs which utilize more sophisticated optical systems than the conventional refractive lens approach. Possibilities include the diffraction optics HUD, the reflective HUD, and the PERI-HUD all of which have unique optical systems and also increased field capabilties. The reflective HUD exhibits a wider field of view than the refractive, but offers no improvement in symbol brightness. The PERI-HUD offers wider field of view but presents a monocular image of the outside world by virtue of its combiner design. The diffraction optics HUD is capable of an even wider field of view than the reflective HUD and also can improve the symbol brightness and viewability with which the pilot sees the outside world. This diffraction optics HUD is described generally in the above patents to Opittek and to Withrington. It uses a holographic optical element (HOE) combiner which both efficiently reflects a generated image emanating from a narrow bandwidth source such as a CRT and provides a higher degree of transmission through the combiner of the light coming from the outside world.
The use of a diffraction optical element for the combiner is a relatively new concept that has developed in the past few years. It has evolved from holographic recording technology; hence the name holographic optical element (HOE). It involves the design and application of lenses based on diffraction principles rather than refraction or reflection as in conventional optics. When utilized in conjunction with conventional lenses, optical systems can be designed with characteristics not otherwise achievable. In the case of the HUD, this provides the designers with the ability to design highly efficient, wide field of view systems, with configurations that are more compatible with cockpit geometry considerations than can be achieved by conventional optics alone. Such systems, however, introduce unique problems of their own.
The present invention enables the large aberrations present in the holographic optical element to be better corrected than has previously been possible. The relay lens design forms disclosed and claimed herein are capable of operating with relay lens fields of view of 40 to 55 degrees or more, while being short and extremely fast (F/0.75 to F/1.5). These design forms permit the design of wide field of view (FOV) holographic head-up displays in which the hologram focal length is short so that the optical system is compact and can be packaged within the restricted space of an aircraft cockpit. Previous HUD designs (a) did not include corrections of aberrations by tilting relay lens elements, and (b) were limited to relay design forms not capable of the wide FOVs and low F-numbers required for improved display configurations.